本论文根据最小均方根误差准则（MMSE）,提出一种非正交设计导频的方法，这种方法在MASSIVE MIMO中非常有效。-In this letter, the error variance of the Minimal

Mean-Square Error (MMSE) channel estimator is analyzed, and

its analytic formula is given. Based on the analytic formula, a

designing criterion on the pilot signals is proposed. When training

time slots can be large enough, our criterion confirms the

well-known fact that orthogonal pilot signals are optimal. But if

training time slots are not enough to support this orthogonality,

our results show that the error variance is lower-bounded away

zero, no matter what kind of pilot signals are used. In this

case, the design of pilot signals should be implemented by using

the line packing on a complex Grassmannian manifold